This invention concerns an improved nonrotating inner tube assembly for dual conduit drill pipe.
As described in Henderson U.S. Pat. No. 3,208,539, dual conduit drill pipe is suitable for use in a wide range of drilling operations. Such pipe includes an outer tube and a concentrically mounted inner tube which cooperate to define two passageways: a central passageway inside the inner tube which may be used to transport cuttings from the drill bit to the surface, and an annular passageway between the inner and outer tubes which may be used to transport drilling fluid from the surface to the drill bit.
In the Henderson dual conduit pipe, the inner tube is positioned inside the outer tube by a series of spacer ribs which are welded to the inner tube and frictionally secured in the outer tube in a light pressed fit. The inner tube is rigidly connected to the outer tube at one end only, as by spot welding the spacer ribs to the outer pipe.
Another means for securing the inner tube to the outer tube is disclosed in Ford U.S. Pat. No. 3,978,923. This assembly employs a spyder or lugs which are welded to the inner tube and rest on a shoulder formed in the outer tube. The spyder is held in place against the shoulder by a snap ring, and the inner tube is free to rotate within the outer tube. This assembly may be used in conjunction with steel lugs which are welded to the inner tube at several places along the length of the inner tube in order to centrally position the inner tube within the outer tube.
Both of these approaches to holding the inner tube within the outer tube suffer from certain disadvantages. In each case, the centering spacers bear against the outer tube, and friction between the spacers and the outer tube tends to rotate the inner tube with the outer tube. However, the rate at which the inner tube tends to rotate varies with the height of the spacers. Given a constant rate of rotation of the outer tube, the inner tube will tend to rotate at differing rates along its length, corresponding to the differing heights of the individual spacers. This effect is particularly acute in crooked bore holes, because such holes cause the outer pipe to bend back and forth as it rotates in the hole. This causes first one centering spacer and then another to engage the inner surface of the outer tube. In some cases the outer tube has been severely worn because of friction with the centering spacers. In other cases, it is the centering spacers which have been worn by the outer tube. On occasion, inner tubes have been observed to break due to torque resulting from a differential between the rates of rotation imposed at two points along the length of the inner tube.